To achieve a long-distance and high-capacity transmission system, a technique for transmitting an optical signal by using digital signal processing has been studied and developed. Optical transmitters using digital signal processing can generate optical signals of a desired modulation format. For example, if an optical signal of a multi-value modulation format (quadrature phase shift keying (QPSK), 16 quadrature amplitude modulation (QAM) to 256QAM, or the like) is generated, the number of bits transmitted for each symbol increases. It is also possible to achieve multicarrier transmission (orthogonal frequency division multiplexing (OFDM), Nyquist WDM, or the like) by using digital signal processing. On the other hand, optical receivers can compensate the waveform distortion of received optical signals by using digital signal processing. A known example thereof is a method for compensating waveform distortion due to chromatic dispersion or the like by using digital signal processing.
A method for monitoring the state of an optical transmission line on the basis of the convergence state of a digital filter in the process of equalizing the waveform of a received signal by using digital signal processing is proposed. The proposed method monitors chromatic dispersion, differential group delay (DGD) between orthogonal polarizations due to polarization mode dispersion (PMD) and polarization dependent loss (PDL), or the like. (For example, Md. Saifuddin Faruk, et al., “Multi-Impairments Monitoring from the Equalizer in a Digital Coherent Optical Receiver”, ECOC 2010, paper Th. 10. A. 1, 2010, and G. Bosco, et al., “Joint DGD, PDL and Chromatic Dispersion Estimation in Ultra-Long-Haul WDM Transmission Experiments with Coherent Receivers”, ECOC 2010, paper Th. 10. A. 2, 2010).
In an optical transmission system, a method for estimating the state of an optical signal by using a known signal has been proposed as relate art. In an optical fiber communication system, a method for measuring polarization mode dispersion from coherent spectrum analysis has been proposed. Furthermore, a monitor circuit for calculating the quality of an optical signal on the basis of equalized electric field data obtained using a digital equalizing filter has been proposed. Furthermore, an optical amplifier repeated transmission system is proposed in which variations in the chromatic dispersion of an optical fiber transmission line due to temperature changes is suppressed. (Japanese Laid-open Patent Publication No. 2010-268390, No. 2004-138615, No. 2009-198364, and No. 8-274713)
Degradation in waveform due to PMD and/or PDL of an optical signal depends on the polarization state of the optical transmission line. However, the polarization state of the optical transmission line is not fixed and changes depending on various factors. For example, the polarization state of the optical transmission line changes depending on the temperature of the optical transmission line, a stress exerted on the optical fiber, and so on. Therefore, it is difficult to determine the worst condition concerning the polarization state of the optical transmission line.
On the other hand, to enhance the reliability of an optical transmission system, the optical transmission system is designed in consideration of the worst condition of the optical transmission line. However, it is difficult to determine the worst condition concerning the polarization state of the optical transmission line, as described above. Therefore, the optical transmission system is sometimes designed with an excess margin. This results in an increase in the number of optical repeaters on the optical transmission line.
Monitoring the polarization state of the optical transmission line over a long period of time allows the worst condition concerning the polarization state to be determined. If parameters for controlling the operation of the optical transmission system are determined on the basis of the thus-determined worst condition, high-quality communication can be achieved. However, it is not preferable that a long period of time be taken to construct the optical transmission system.